1. Field of the Invention
The present invention relates to propylene polymerization. In particular, the present invention concerns a process for preparing alloys of homopolymers and copolymers of propylene in a reactor system comprising a combination of at least one slurry reactor and at least one gas phase reactor.
2. Description of Related Art
A large number of processes for preparing propylene homo- and copolymers are known in the art. Thus, for example when MgCl2*TiCl4, a conventional, supported high-yield catalyst, is used for polymerization, numerous different kinds of bulk and gas phase processes can be employed. The bulk process is a slurry process, wherein the reaction takes place in pure monomer or in a reaction medium containing more than 60 wt-% of the monomer. The method is very useful because of high productivity, and no inert hydrocarbon intermediate is needed. The gas phase processes have lower activity, but instead their investment costs are lower in particular because no special recovery is needed for monomer recycling. The operation of gas phase processes is safer due to low monomer inventory and low pressure. A further advantage is the possibility to produce high comonomer content products.
In following, the various features of bulk and gas phase processes will be discussed in some detail.
Polymers formed during bulk polymerization, in particular polymers of high comonomer content and/or low molar mass or low isotacticity, are soluble in the reaction medium. This causes considerable problems in bulk/slurry polymerization applications. Many of these problems can be solved as described in, e.g., FI Patent Application No. 906427 (WO 92/12181) and 906428 and (WO 92/12182) and International Patent Application WO 92/12181 by using super critical slurry processes where the reaction medium is under critical temperature and pressure. By nature the super critical fluid has lower polymer solving power, and nearly unlimited solubility of gaseous components, e.g. hydrogen. Simultaneously, the separation of the recycled reaction medium and recovered polymer is simplified, because of the energy available in the polymerization product.
In case of a bulk reactor, recovery and recycling of the unreacted monomer is always needed for economic operation, because up to 50% of the mass of the slurry is liquid. Usually, the monomers are separated from the polymer of the polymerization product in a flash system. In practice, the polymerization product from the reactor outlet is heated and expanded in a flash pipe and/or flash tank in order to evaporate the reaction medium. The evaporated reaction medium is separated for recovery e.g. by condensing. The flash pipe can be jacketed and heated preferably by steam Hydr. Carb. Proc, November 1984 p. 113-117, DiDruscoandRinaldi.
There are some problems related to conventional flash systems. Thus, any non-evaporated liquid in the separation tank is a risk for blocking the device. The flash system is highly sensitive to high soluble polymer fractions, e.g. with high comonomer content or low molar mass, but also to fines in the polymer product. This is particularly true for cyclone type of devices operated at high pressures.
The recovery of the monomer can be simplified by condensing the off gas with cooling water Hydro carb. Proc., November 1984 p. 113-117, DiDruscoandRinaldi. Thus, the off gas has to be pressurised with a compressor, or even more advantageously, by using high enough off gas separation pressure. The hydrogen concentration is not limited in gas phase reactors. The gas phase reactor also provides flexible production rate control because the bed level and monomer pressure can be controlled.
In a fluidized bed reactor the heat transfer is better than in a mechanically fluidized reactor but inferior to the heat transfer in a bulk loop reactor. The cooling of the fluid bed reactor is effective due to high circulation flow of reactor gas through the heat transfer, and it can be further improved by cooling the circulation gas below the dew point of the gas, cf. EP 0 089 691, U.S. Pat. Nos. 4,543,399, 4,588,790, WO 96/04321. Conventionally up to 20 wt-% of the circulation stream can be condensed, and with a special control arrangements up to 50 wt-% is possible, cf. U.S. No. 93/03,946, WO 94/25495. It is also possible to separate the condensed monomer and light hydrocarbons in a knock-out drum, and feed it back to the fluid bed reactor with special nozzles, GB 94/01074, WO 94/28032.
None of the above-described processes is particularly well suited for the production of different kinds of homo- and copolymers of propylene.
In order to draw benefit from the different advantages of the slurry bulk and the gas phase processes, a number of combined bulk and gas phase processes have been suggested in the prior art.
For polymerization of vinyl chloride, there has been described in the prior art a slurry/gas phase reactor cascade where the first reactor is a loop reactor, and the content of the loop reactor is lead to a second reactor, which is a fluidized bed reactor (cf. U.S. Pat. No. 3,622,553). The polymerization is continued in the fluidized bed.
Polymerization of propylene in a loop reactor, which can be operated in supercritical conditions, is disclosed in U.S. Pat. No. 4,740,550. The product of the loop reactor is conducted to a gas phase reactor, wherein the reaction is continued. Before entering the gas phase the fines fraction of the polymerization product of the loop reactor is removed and circulated back to the loop reactor. Together with the fines, a part of the monomers are recycled. The loop reactor is merely used for prepolymerization because the loop reactor residence time has to be short  less than 30 min. Also feeding the fines back to the loop reactor will eventually fill up the loop reactor with inactive fines. The preferred embodiment of the prior art process comprises preparing 50 to 95%, in particular 70 to 90%, of the product in the loop. Further, the extensive recycling from the gas phase reactor back to the loop is arranged by condensing. As a result no product having broad or bimodal molar mass distribution can be prepared.
For polymerization of olefins a process is known in which the first reaction is made in liquid, and the second reaction in the absence of the liquid, GB Patent No. 1 532 231.
A two-step process has also been suggested for polymerization of ethylene, cf. U.S. Pat. No. 4,368,291.
A slurry prepolymerization connected to the gas phase process is proposed in WO 88/02376.
A gas phase process for polyolefins, where a special catalyst with spherical form is employed, has been proposed in EP-A 0 560 312 and EP-A 0 517 183. The catalyst is prepolymerized in a loop reactor using more than 5 parts polymer and 1 part catalyst up to 10% (w/w) of total production.
Some of the disadvantages associated with the loop and gas phase reactors, respectively, are avoided by the suggested prior art processes. However, none of them meets the requirements for flexibility and low production costs dictated by the production of large varieties of polyolefin qualities using one and the same process configuration. The recycling of rather large amounts of unreacted monomers to the slurry reactor impairs the loop reactor dynamics and slows up the transition to novel product qualities.
It is an object of the present invention to eliminate the problems related to the prior art of single and multiple-reactor processes and to provide a novel process for preparing homopolymers and copolymers of propylene.
It is another object of the invention to provide a highly versatile process which can be used for preparing a wide range of different homopolymer and copolymer products of propylene, in particular alloys of homopolymer and random copolymers optionally also having very different molar masses.
It is a further object of the invention to provide for the production of conventional homo- and copolymers, and to provide a process enabling tailoring of their molar mass distributions and comonomer distribution.
These and other objects, together with the advantages thereof over known processes, which shall become apparent from specification which follows, are accomplished by the invention as hereinafter described and claimed.
The process according to the present invention is based on the use of a combination of at least one slurry reactor and at least one gas phase reactor connected in series, in that order, to form a cascade. Propylene homo- and copolymers are prepared in the presence of a catalyst at elevated temperature and pressure. According to the invention, the reaction product of at least one slurry reactor is subjected to product separation by reducing the pressure thereof to evaporate volatile components. The solid substances of the product separation operation are conducted to a first gas phase reactor. The evaporated reaction medium including the unreacted monomers are separated from the other volatile components and also fed to a gas phase reactor, whereas hydrogen and inert hydrocarbons (e.g. lower alkanes), if any, are removed.
More specifically, the process according to the present invention is mainly characterized by (a) polymerizing propylene optionally with comonomers in the presence of a catalyst at an elevated temperature of at least 60 C. and at an elevated pressure in at least one slurry reactor and at least one gas phase reactor, at least 10% of the polymer product being produced in the gas phase reactor(s); (b) recoverying from the slurry reactor a polymerization product containing unreacted monomers; (c) separating volatile components from the product in a separation unit (d) recovering the polymerization product from the separation unit and conducting it to a first gas phase reactor, and (e) recovering at least a part of the volatile component from the separation unit and conducting them to a gas phase reactor essentially without recycling unreacted monomers to the slurry reactor.
The invention achieves a number of considerable advantages. With the present arrangement it has been found that the monomer fed into the first reactor can, to a large extent or fully, be consumed in the gas phase reactor(s) after the slurry reactor. This is possible due to gas phase operation with high gas hold-up and a minimum of off-gases. The loop reactor dynamics in the cascade provides fast transitions. Fast start-ups are also possible because the gas phase bed material is available directly from the loop reactor. With the loop and gas phase reactor cascade it is possible to produce a large variety of different bimodal products. The at least one gas phase reactor provides high flexibility in the reaction rate ratio between the first and second part of the product because of adjustable bed level and reaction rate. Further, the gas phase reactor having no solubility limitations makes it possible to produce polymers of high and very high comonomer content or, alternatively, very high melt flow rate.
The separation of light components before feeding of the recovered monomer to the gas phase makes it possible independently to control polymerization conditions and provides maximum flexibility for polymer alloy preparation.